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Abstract

Background

There is current interest in the role of perinatal factors in the aetiology of diseases
that occur later in life. Infectious mononucleosis (IM) can follow late primary infection
with Epstein-Barr virus (EBV), and has been shown to increase the risk of multiple
sclerosis and Hodgkin's disease. Little is known about maternal or perinatal factors
associated with IM or its sequelae.

Methods

We investigated perinatal risk factors for hospitalised IM using a prospective record-linkage
study in a population in the south of England. The dataset used, the Oxford record
linkage study (ORLS), includes abstracts of birth registrations, maternities and in-patient
hospital records, including day case care, for all subjects in a defined geographical
area. From these sources, we identified cases of hospitalised IM up to the age of
30 years in people for whom the ORLS had a maternity record; and we compared perinatal
factors in their pregnancy with those in the pregnancy of children who had no hospital
record of IM.

Results

Our data showed a significant association between hospitalised IM and lower social
class (p = 0.02), a higher risk of hospitalised IM in children of married rather than
single mothers (p < 0.001), and, of marginal statistical significance, an association
with singleton birth (p = 0.06). The ratio of observed to expected cases of hospitalised
IM in each season was 0.95 in winter, 1.02 in spring, 1.02 in summer and 1.00 in autumn.
The chi-square test for seasonality, with a value of 0.8, was not significant.

Other factors studied, including low birth weight, short gestational age, maternal
smoking, late age at motherhood, did not increase the risk of subsequent hospitalised
IM.

Conclusions

Because of the increasing tendency of women to postpone childbearing, it is useful
to know that older age at motherhood is not associated with an increased risk of hospitalised
IM in their children. We have no explanation for the finding that children of married
women had a higher risk of IM than those of single mothers. Though highly significant,
it may nonetheless be a chance finding. We found no evidence that such perinatal factors
as birth weight and gestational age, or season of birth, were associated with the
risk of hospitalised IM.

Background

Infectious mononucleosis (IM) can follow a pathologically strong immune response to
primary infection with Epstein-Barr Virus (EBV), mainly during adolescence or young
adulthood[1]. The majority of individuals have a primary infection with EBV during infancy and
childhood, and are asymptomatic or only experience a mild clinical course[2,3]. Some individuals who are not infected in childhood are subsequently infected in
adolescence or adulthood, leading to more severe disease. For example, a recent study
of university students in Edinburgh found that three-quarters were EBV seropositive
at entry to university; that, of the quarter who were seronegative, almost half experienced
EBV sero-conversion over the following three years; and that, of these, 25% developed
IM[4]. Two-thirds of the IM cases, but only one tenth of the asymptomatic primary EBV infections,
were statistically attributable to sexual intercourse[4]. At least in student populations, sexual intercourse and intimate kissing are important
factors in the transmission of EBV infections that lead to IM.

Epidemiological and laboratory data show that EBV infection is also associated with
several other diseases. EBV was first identified in Burkitt's lymphoma cells[5]. EBV infection has been known for many years to be associated with nasopharyngeal
carcinoma in some parts of the world, [6] and with Hodgkin's disease (HD),[7-9] associations that have been amply confirmed[3]. More recently, it has been demonstrated that IM is a risk factor for multiple sclerosis
(MS)[10-13]. Factors that are relevant to the epidemiology of IM may also have some relevance
to the epidemiology of Hodgkin's disease and MS, and vice versa.

There has been considerable interest in recent years in the influence of maternal
and perinatal factors on the subsequent development of disease in later life[14]. Much of the interest has focused on subsequent chronic non-infectious diseases,
such as hypertension, coronary heart disease and diabetes,[15-17] rather than acute infectious disease. Specifically, there is little or no information
on whether perinatal factors might have any influence on the development of IM.

There are reasons to consider the possibility that perinatal and/or other early life
factors might influence the risk of IM. First, there is the fact that many individuals
are infected with EBV very early in life, while others are not and have an increased
risk of IM later. Second, Purtilo and Sakamoto reported that reactivation of EBV commonly
occurs in normal pregnant women and commented that "the impact of pregnancy on outcomes
of EBV infections has not been thoroughly evaluated" in respect of either the mother
or child[18]. There is still a paucity of research in this area. Third, migration patterns for
MS, between high and low risk countries, show that the risk of MS is substantially
determined by place of residence in early life rather than later[19,20]. Fourth, there are reasons to think that pregnancy-related or other early life factors
may influence the development of MS in some people: in particular, there is increasingly
strong evidence that the distribution of season of birth in people with MS differs
from that in the general population[21,22]. There is an excess of spring births, albeit a numerically modest excess, among people
with MS with the implication that pregnancy-associated factors may be relevant to
the risk of MS. There is also some evidence of season of birth effects in HD with
a slight excess of spring births in young people with HD[23].

For these reasons, we decided to use the Oxford record linkage study (ORLS) to study
perinatal factors in people who developed IM, as part of a wider programme of work
studying the influence of perinatal factors on the subsequent development of disease
in the offspring[24-26]. The ORLS dataset has already been used, in previous studies, to demonstrate that
there is an increased risk of MS and of HD in individuals following admission to hospital
with IM in the Oxford area[27,28].

Methods

The Oxford record linkage study (ORLS) includes abstracts of birth registrations,
maternities and in-patient hospital admission records, including day case care (ie
admission to hospital for care without overnight stay), for all subjects in a defined
geographical area of South East England. The maternity data covered all National Health
Service (NHS) hospitals in two health districts from 1970 to 1989 (in 1989 detailed
data collection on maternity in the ORLS stopped after reforms by the government to
increase the uniformity of NHS data collection systems). Cases of hospitalised IM
were identified using in-patient and day case admission data in the ORLS for all clinical
specialties and from all districts covered by the ORLS including those that did not
collect maternity data. These data covered the two health districts from 1970 to 1999
(population 0.9 million in 1999); a further four adjacent districts from 1970-1991
(total population 1.9 million); and all eight districts of the former Oxford region
from 1991-1999. The maternity data were extracted from maternity records by clerical
staff, trained at the ORLS by senior medical staff. In the 30-year period covered
by this study, the abstracts relating to the same individual were linked as part of
the Oxford region's NHS health information system. Similarly, the records of each
mother and her offspring were routinely linked. From these sources, we identified
cases of hospitalised IM up to the age of 30 years in people for whom the ORLS had
a maternity record. IM in the mothers was identified by record linkage of each mother's
maternity record to hospital admissions for the mother before and after the pregnancy
from 1963 to 1999.

Exclusions from the analysis of the maternity dataset included 985 abortions, 1560
stillbirths and 1567 neonatal deaths within 30 days of birth. In 289 maternities,
the birth weight was recorded as less than 1000 g - these were also excluded because
most of these records had implausibly low values and/or missing data for many of the
risk factors investigated. None of these 289 excluded babies had a subsequent record
of IM. After exclusions, records of 248 659 children remained. Some data items such
as social class, mother's smoking and breast feeding at the time of discharge from
hospital were not collected until 1975.

We accepted, as a case of IM, each person in the ORLS who had a hospital discharge
record that included the International Classification of Diseases (ICD) code for IM.
The codes used were 075 in the eighth and ninth revision of the ICD and B27 in the
tenth. The occupation of the head of the mother's household was recorded based on
husband's occupation, or the mother's occupation if single and working, or the mother's
father's occupation if not. It was recorded contemporaneously on the mothers' hospital
records, obtained by trained interviewers at hospital admission; and was subsequently
coded by trained coders as occupational social class in the five standard groups then
used in English national statistics (social class one is the most advantaged socio-economic
group, and social class five the most deprived).

The duration of follow-up for the offspring ranged from 30 years for those born in
1970 to 10 years for those born in 1989, with a mean follow-up duration of 18 years.
We analysed all cases of IM together and then split the analysis into those aged 10
and under and those aged 11 years and over. We did this, first, because we have 10
years of follow-up of all infants, but a variable length of follow-up thereafter;
and, second, because we were particularly interested in cases of late onset IM.

Statistical methods used include chi-squared tests to assess the significance of associations
between each individual perinatal risk factor and IM in the offspring, and logistic
regression modelling to investigate risk factors that had significant independent
associations with IM. Statistical significance was measured at the standard 5% level.
When using logistic regression, all variables that were significant (P < 0.05) in
the univariate analysis were included in the initial model and the variables that
were not significant were removed before running the initial model. Thereafter, each
of the variables that were not significant in the univariate analysis was re-introduced
into the model, one at a time. The purpose of this was to test whether any variable,
if not significant in univariate analysis, became significant when modelled with other
significant variables. Missing data were excluded only for those terms that were included
in the logistic regression model.

In order to test the hypothesis that the season-of-birth distribution for people with
IM may differ from that in the general population, we also used a much larger ORLS
dataset. This dataset covers all records in the ORLS from 1963-1999, not just those
linked to maternity records in the smaller area from 1970-1989. We analysed season
of birth in patients who were born in the UK (to avoid confounding with the place
of birth of people born overseas, e.g. on the Indian subcontinent). We calculated
the 'expected' number of births of IM patients in each month by applying the monthly
distribution of all births in the general UK-born population in the ORLS to the number
of people with IM. We did this with adjustment for year of birth, sex, and for differences
in the number of days in different months. We compared the expected number of births
in each month with the observed number, and expressed the result as a ratio of monthly
observed to expected. We used a chi square test for heterogeneity to test for differences
between individual months and between four seasons of winter (December, January, February),
spring (March, April, May), summer (June, July, August) and autumn (September, October,
November).

To provide contextual information on the incidence of hospitalised IM in the region
covered by the study, we analysed trends over time in population-based admission rates
using the whole ORLS dataset.

The English NHS Central Office for Research Ethics Committees approved the current
work programme of analysis using the linked dataset (reference number 04/Q2006/176).

Results

There were 225 people with a maternity record in the ORLS and with a subsequent admission
for IM. 69 of the 225 people with hospitalized IM (31%) were aged 10 years or less
at the time of admission for IM (39 males, 30 females); and 156 (69%) were aged 11
and over (74 males, 82 females). We noted that, although there were more male than
female cases in the age group under ten, and more females than males in the age group
aged 10-19, these findings did not reach statistical significance. Numbers of people
admitted for IM were highest in the 15-19 age group, and a larger number of over 20s
were admitted than children under 5 (Table 1). This age profile of patients contrasts with that of EBV infection, generally, which
frequently occurs in infancy.

Table 1. Number of people admitted to hospital for infectious mononucleosis (IM), based on
age at admission and sex.

There was no significant association between IM in the child and maternal IM, smoking,
parity, ABO blood group and rhesus status (Table 2). In most analyses, we grouped parity as either first-born or subsequent-born. However,
we show parity in greater detail in Table 3 to demonstrate that there were no important differences, in detail, between those
with and without IM. Overall, IM was more common in children of younger than of older
mothers; the same was observed in those with IM aged 11 and over, though differences
were marginally significant (p = 0.04) and fairly small (Table 2). There was an increased risk of IM amongst lower social classes though, again, the
differences were fairly small (Table 2). IM was significantly more common in children of mothers who were married than in
children of mothers who were single (p < 0.001): ninety seven per cent of children
admitted with IM had a married mother compared with 90% of children who were not admitted
with IM (Table 2).

Table 2. Associations between mothers' characteristics and IM in the child

Table 3. Distribution of mothers' parity at the time of birth of people without and with eventual
IM

Children with IM were more likely to have been singletons than others, but this finding
did not quite reach statistical significance (P = 0.2 overall, 0.06 among those aged
11 years and over, Table 4). We found no significant association between IM and birth weight, gestational age,
breastfeeding, caesarian birth, presentation at delivery or Apgar scores at 1 and
5 minutes after delivery. Children with IM were significantly more likely to have
had a forceps delivery than a child without IM, both in the all-ages analysis (p =
0.008) and in that for children with IM aged 11 years and over (p = 0.02). There was
a borderline significant association between pre-eclampsia and IM (p = 0.07) (Table
3).

Table 4. Associations between characteristics of the births and IM in the child

An association with marital status persisted after multivariate adjustment: IM was
less common in children of single mothers than in children of married mothers (odds
ratio, single to married, 0.36, 95% confidence interval 0.16-0.80) after adjustment
for maternal age, parity and social class. The association between marital status
and IM seems to be independent of either parity or social class, and is illustrated
in Table 5. As Table 5 shows, the percentages of children with IM are systematically higher for those whose
mothers were married, regardless of parity (summarised as first-born or subsequent
child) and regardless of social class (summarised as 1 and 2, the most favoured social
class, to 4 and 5, the most deprived). However, numbers of cases of IM in the unmarried
category are very small, and, though the differences were systematic they were not
generally statistically significant within subgroups.

Table 5. Percentage in each group with IM, by marital status, and numbers on which percentages
are based (n/N)

Multivariate analysis showed that pre-eclampsia and use of forceps during delivery
were not independently associated with an increased risk of IM, after controlling
for year of birth and social class.

In the analyses of season of birth, there was no significant association in the dataset
of the 225 patients on whom we had a maternity record. In the full ORLS dataset (see
Method), there were 1695 people with a record of hospitalised IM. The ratio of observed
to expected cases of IM in each season was 0.95 in winter, 1.02 in spring, 1.02 in
summer and 1.00 in autumn. The chi-square test for seasonality, with a value of 0.8,
was not significant.

The analyses of trends show that there were no major changes over time - for example,
admission rates in the ORLS area were 4.4 per 100,000 population in 1975, 4.4 in 1985
and 4.7 in 1995.

Discussion

Strengths and limitations

Strengths of this study are that data collection was prospective, undertaken in a
large and well-defined population, over a period of 30 years, including around 250,000
births, and recall biases are impossible. Data concerning perinatal risk factors,
and subsequent IM, were collected independently. They were brought together by record-linkage,
and therefore data about risk factors could not have been influenced by knowledge
of the study outcome (IM) or by the kinds of interviewer, recall or attribution bias
that can handicap case-control studies based on interviewing patients.

Despite the large study population, the total number of cases of IM identified was
a modest 225. This limits the power of the study. To our knowledge, there are no other
reports in the published literature concerning perinatal factors and subsequent IM.
Cases of IM not requiring hospitalisation will have been missed by this study. IM
is diagnosed primarily based upon a clinical picture of symptoms, peripheral blood
smear, and heterophile (Monospot) antibody test. It seems likely that hospitalised
cases are more likely than those that do not warrant admission to have had confirmatory
tests done to establish the diagnosis with certainty. However, we do not have data
on the diagnostic criteria used in, or the clinical features of, the study population.
We had to accept a coded diagnosis on the hospital discharge abstract. Current privacy
regulations preclude checking the actual medical records of the patients for further
detail.

There are some gaps in the data collection: smoking behaviour and social class were
not routinely collected for a few years of the study. We could not identify records
of children who were diagnosed with IM after moving away from the ORLS region, lowering
our observed incidence of IM. It is certain that our observed IM incidence is lower
than the true incidence of IM. However, the influence of perinatal risk factors, when
comparing children with and without IM, should not be biased unless migration itself
is associated with both the risk of subsequent IM and putative perinatal risk factors.
We found very few significant associations. It is theoretically possible, though we
think unlikely, that associations have been missed as a result of unmeasured confounding,
i.e. that a true association has been masked by confounding factors that act in equal
and opposite directions to a true cause-and-effect association.

Although cases of IM requiring hospital admission are infrequent, they are likely
to represent people at the severe end of the clinical spectrum. If perinatal and maternal
factors affect the risk of IM, they are more likely to affect those with severe disease.
Those with symptoms severe enough to warrant hospital admission may also have the
strongest reactions to primary EBV infection, which in turn, may represent individuals
who are more susceptible to diseases where EBV is thought to play an aetiological
role, notably HD and MS[29]. We hope that others will be stimulated to publish results from similar databases
and, if future individual studies are limited in size, we hope that our data and others
could be pooled to produce meta-analyses.

Delayed EBV infection, IM, HD and MS

In the majority of individuals, primary EBV infection occurs during early childhood
and is often asymptomatic, but delayed EBV infection may result in IM in adolescents
and adults. The symptoms of IM, most notably fever, sore throat, swollen glands and
fatigue, are thought to be the clinical manifestation of an exaggerated T cell response
to EBV infection and the release of inflammatory cytokines[30]. It has been suggested that the size of the initial viral dose of EBV may be a contributing
factor in the development of IM and that adolescents may be more likely to encounter
a larger viral dose through deep kissing during penetrative sexual intercourse[4]. A relationship between the level of the T cell response and the severity of IM has
also been noted[31]. The difference in severity of symptoms between those infected with EBV at a young
age and those infected during adolescence and early adulthood may be the difference
in magnitude of the viral dose, with a smaller dose acquired by salivary contact in
children than that acquired through sexual contact in adolescents and young adults[4,32]. In addition, recent genetic markers in the HLA class I locus have also been implicated
in the immune-response to EBV infection in both IM [33] and HD,[7] suggesting that genetic factors may also play a role. Immunopathological mechanisms
involved in IM, contrasted with those in asymptomatic primary EBV infection, have
been reported[32,34]. As our findings may only be representative of cases severe enough to require hospital
admission, further studies in people with IM who have not been hospitalised may be
beneficial.

EBV-positive Hodgkin's disease has been found to be more common in people with a previous
diagnosis of IM,[29] and an almost 100% prevalence of EBV seroconversion has been found in MS patients,
as compared to a 90% seroconversion rate in the general population[34]. There is growing evidence of associations between IM and both HD [7-9] and MS[10-13]. The 'hygiene hypothesis' has been put forward as a possible explanation for a causal
pathway between EBV and HD and MS. It proposes that a lack of early life infections
or exposure to viral pathogens in childhood may prevent the normal processes of immune
maturation, leading to increases in rates of both allergic and immune-mediated conditions,
such as MS[35]. Perinatal and early life factors that may affect late exposure to infection may
play a role in the relationship between these conditions.

Principal findings

The lack of association between increasing maternal age and hospitalised IM found
in the current study is important, given the trend in Western countries towards postponement
of childbearing. It is now common for women to give birth well into their late 30s
or early 40s, and it is reassuring that older motherhood does not seem to carry an
increased risk for IM. However, although maternal age has increased over recent years,
in the years covered by the study (1970-1989), most mothers were under 35 (94% in
our data).

There was no association between season of birth and hospitalised IM.

Our data suggested that pre-eclampsia, and forceps delivery, had a borderline significant
association with subsequent IM. There was no residual association after controlling
for other factors, suggesting that confounding was responsible for these apparent
associations.

Children born as one of a pair of twins had a borderline significant lower risk of
developing IM (p = 0.06) than that of singletons. If this is not a chance finding,
it would support studies proposing that increased sibship sizes can protect against
IM (and its long term sequelae, including MS), by exposing children to viral infections
early in life[36]. The reasoning, part of the hygiene hypothesis, is that children born as one of twins
are more likely to be exposed to EBV infection early in life, through physical and
salivary contact with their sibling, thus reducing their risk of delayed EBV infection,
and therefore IM, later in life.

There is no information in the literature about marital status and IM, or delayed
childhood EBV infection. Our results show that children born to single mothers had
a significantly lower risk of hospitalised IM than those born to married mothers.
We have no explanation for this, although one possibility is that (for a given level
of severity of illness) single mothers may have had greater difficulty than married
mothers in accessing hospital care. Though possible, we think that this is unlikely
in that, with free access to National Health Service care, children deemed to be in
need of hospital care are likely to have received it. It is possible that, though
the finding was highly statistically significant, it may nonetheless have arisen from
the play of chance. It is worth noting that, in the era of the pregnancies covered
by this study, single motherhood was much less common in England than it is now. Previous
studies have found clustering of infectious diseases within households in which an
older child is present[36]. Although parity is an incomplete measure of contact with older children within the
household, it was the only measure available to us. It did not come close to significance
in this study. It is unlikely that the association with single mothers is confounded
by parity: it persisted after adjustment for parity and, in any case, there was no
association between parity and risk of IM (Tables 2, 3).

We found a modest association between IM and lower social class. It is generally held
that, if anything, IM is a little more common in higher social classes[36]. However, the patients in our study are those admitted to hospital and it is possible,
even likely, that typical clinical thresholds for admission of patients with IM can
be influenced by patients' socio-economic circumstances. Thus, for a given level of
clinical severity, it is possible that children in less favoured socio-economic circumstances
may be more likely than others to be admitted to hospital. The literature is conflicting
over the relationship between social class and possible sequelae of late infection
with EBV and HD. Several studies have reported minimal or no effect of social class
on MS [37,38] or HD[39,40]. It has also been reported that EBV-infection-associated HD is in fact more common
in lower social classes,[41] although this association only reached statistical significance in females[41].

Conclusion

In summary, the association with single motherhood deserves further study, as does
the possibility that reduced contact between young children may increase the risk
of IM and possibly, for a few, eventually the risk of MS or HD. Other perinatal factors
studied by us, including season of birth, were not associated with an increased risk
of hospitalised IM. Of some importance, late age at motherhood was not a risk factor.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

MJG designed the study, with input from IM and OA-M. CJW undertook the analyses. All
authors contributed to the interpretation and discussion of findings. IM wrote the
first draft and all authors contributed to the final manuscript.

Acknowledgements

Over many years, the linked datafiles were built by Leicester Gill, Glenys Bettley
and Myfanwy Griffith. The Unit of Health-Care Epidemiology is funded by the English
National Institute for Health Research. The views expressed in this paper do not necessarily
reflect those of the funding body.